1. Field of the Invention
The present invention relates to a solid-state laser apparatus, and in particular to a solid-state laser apparatus for use in an optical pick-up and an optical printer apparatus.
2. Description of the Related Art
In recent years, the cost of a high-power semiconductor laser (e.g. laser diode) has decreased and with research the development and product design of a solid-state laser apparatus using a semiconductor laser has become prosperous. A solid-state laser apparatus using an exciting type semiconductor laser has a very high efficiency because of a smaller spectrum width of an exciting source. In addition, since the laser exciting source is small, the solid-state laser apparatus can also be miniaturized. Further, with the solid-state laser apparatus, a successive oscillation of high power and a high quality beam at room temperature can be realized. In addition, the solid-state laser apparatus is superior in an accumulative property of energy and a stability of frequency.
FIG. 5 shows a structure of a background semiconductor laser exciting Q switch-SHG (Second Harmonics Generation) solid-state laser apparatus. The solid-state laser apparatus 500 includes a semiconductor laser 501 used for excitation, a focusing lens group 502, a rear mirror 503, a laser crystal 504, a polarizing plate 505, an optoelectronic crystal 506 for use in a Q switch, an output mirror 507 and a two-times wave (i.e., twice the frequency of a fundamental frequency) generating non-linear optical crystal 508.
In order to realize a Q switch operation, the Q switch optoelectronic crystal 506 is inclined by 45.degree. from the direction of the polarizing plate 505 in relation to an advance phase axis and a delay phase axis of the crystal.
Further, electrodes (not shown) for applying a high voltage pulse are provided on upper and lower surfaces of the Q switch optoelectronic crystal 506. When the high-voltage pulse is not applied to the Q switch optoelectronic crystal 506, the optoelectronic crystal 506 operates as a 1/4 wavelength plate (hereinafter referred to as a .lambda./4 plate). On such occasion, since the Q value of the resonator can be limited to a small value, laser oscillation is not performed.
On the other hand, in the case of applying the high-voltage pulse to the Q switch optoelectronic crystal 506, the Q switch optoelectronic crystal 506 operates as an O-wavelength plate. Then, the Q value of the resonator is large and thereby laser oscillation is performed. Consequently, a high-peak voltage output can be realized. Furthermore, a reflection coefficient (reflectance) of the output mirror 507 can be set to a state of almost total reflection for a laser oscillation fundamental wave, and the reflectance of the output mirror 507 can also be set to a state of almost total transmission for a wave with a frequency two times that of the laser oscillation fundamental wave (hereinafter referred to as a two-times wave). Therefore, the non-linear optical crystal 508 can effectively generate a high-peak output 510 of the two-times wave.
However, the solid-state laser apparatus 500 discussed above has a problem in that the number of parts needed is large and energy loss occurs due to controlling the direction of polarization with the polarizing plate 505. With a large number of parts, the apparatus becomes larger in size and more expensive.
The Japanese Laid-open Patent Publication No.6-088879/1994 discusses a solid-state laser apparatus employing a Q switch-SHG complex element to solve such a problem. FIG. 6 shows the construction of a solid-state laser apparatus discussed in this publication.
The solid-state laser apparatus 600 includes of a semiconductor laser 501 used as an exciter, a focusing lens 502, a laser crystal 621, a Q switch-SHG non-linear optical crystal 622, and an output mirror 507. The laser crystal 621 includes a Nd:YVO.sub.4 crystal, and a KTP (KTiOPO.sub.4) crystal is included in the Q switch-SHG non-linear optical crystal 622.
The construction of the solid-state laser apparatus 600 is similar to that of the above-mentioned laser apparatus 500. However, the effect of the polarizing plate 505 (Refer to FIG. 5) is created by the laser crystal 621 and the effect of the Q switch optoelectronic crystal 506 (Refer to FIG. 5) is created by the Q switch-SHG non-linear optical crystal 622. Consequently, the number of the employed parts can be reduced, and therefore the size and cost of the apparatus can be reduced.
In more detail, the laser crystal 621 realizes a straight polarization output by utilizing a difference of an inductive radiation cross-sectional square measure in an a-axis direction and a c-axis direction of the Nd:YVO.sub.4 crystal. Namely, the inductive radiation cross-sectional square measure in the c-axis direction of the Nd:YVO.sub.4 crystal is almost four times that of the a-axis direction. The laser oscillation of straight polarization is performed by the difference therebetween. Consequently, in the device of FIG. 6 it is not necessary to include the polarizing plate 505 (as shown in FIG. 5).
In addition, since the KTP (KTiOPO.sub.4) crystal included in the Q switch-SHG non-linear optical crystal 622 not only operates as the SHG crystal, but has an optoelectronic effect, it is possible to realize the complex operation of the SHG and the Q switch. Consequently, the Q switch element does not need to be prepared individually, and thereby the apparatus can be further miniaturized.
However, the solid-state laser apparatus 600 has a problem in that the maximum output is approximately 230 W, which is insufficient as a Q switch laser light source (see Applied Optics, 35, 4298-4301 (1995)). The reason for this is that the fundamental wave of the oscillating laser becomes longitudinal multiple mode, and thereby the Q switch effect is lowered. Even though the Q switch-SHG non-linear optical crystal 622 utilizes the optoelectronic effect of the KTP crystal, the fundamental wave of the laser is longitudinal multiple mode resulting in a phenomenon in which the wavelength of the laser fundamental wave has an effect of "hopping" on a rise-time of the high-voltage pulse. This hopping is due to a wavelength dispersion effect of the KTP crystal and becomes prominent with longitudinal multiple mode waves. As a result, a peak output is reduced. This problem can be solved by setting the rise-time of the Q switch high-voltage pulse to 0.about.1 ns. However, the cost for such a power source is high.
Further, there exists an influence on an up-conversion process due to the high-powered two-times wave incident on the laser crystal. Semiconductor laser light rays (having a wavelength of 809 nm) are directed as incident light rays in order to excite the fundamental wave thereof. However, the fundamental wave of the laser is unexpectedly excited more or less, even with the two-times laser light rays (having the wavelength of 532 nm).